Plasticizers are incorporated into a resin (usually a plastic or elastomer) to increase the flexibility, workability, or distensibility of the resin. The largest use of plasticizers is in the production of “plasticized” or flexible polyvinyl chloride (PVC) products. Typical uses of plasticized PVC include films, sheets, tubing, coated fabrics, wire and cable insulation and jacketing, toys, flooring materials such as vinyl sheet flooring or vinyl floor tiles, adhesives, sealants, inks, and medical products such as blood bags and tubing, and the like.
Other polymer systems that use small amounts of plasticizers include polyvinyl butyral, acrylic polymers, nylon, polyolefins, polyurethanes, and certain fluoroplastics. Plasticizers can also be used with rubber (although often these materials fall under the definition of extenders for rubber rather than plasticizers). A listing of the major plasticizers and their compatibilities with different polymer systems is provided in “Plasticizers,” A. D. Godwin, in Applied Polymer Science 21st Century, edited by C. D. Craver and C. E. Carraher, Elsevier (2000); pp. 157-175.
The most important chemical class of plasticizers is phthalic acid esters, which accounted for about 84% worldwide of PVC plasticizer usage in 2009.
Others are esters based on cyclohexanoic acid. In the late 1990's and early 2000's, various compositions based on cyclohexanoate, cyclohexanedioates, and cyclohexanepolyoate esters were said to be useful for a range of goods from semi-rigid to highly flexible materials. See, for instance, WO 99/32427, WO 2004/046078, WO 2003/029339, U.S. Patent Publication No. 2006-0247461, and U.S. Pat. No. 7,297,738.
Others also include esters based on benzoic acid (see, for instance, U.S. Pat. No. 6,740,254) and polyketones, such as described in U.S. Pat. No. 6,777,514; and U.S. Patent Publication No. 2008-0242895. Epoxidized soybean oil, which has much longer alkyl groups (C16 to C18), has been tried as a plasticizer, but is generally used as a PVC stabilizer. Stabilizers are used in much lower concentrations than plasticizers. U.S. Patent Publication No. 2010-0159177 discloses triglycerides with a total carbon number of the triester groups between 20 and 25, produced by esterification of glycerol with a combination of acids derived from the hydroformylation and subsequent oxidation of C3 to C9 olefins, having excellent compatibility with a wide variety of resins and that can be made with a high throughput.
For example, in an article entitled “Esters of diphenic acid and their plasticizing properties”, Kulev et al., Izvestiya Tomskogo Politekhnicheskogo Instituta (1961) 111, disclose that diisoamyl diphenate, bis(2-ethylhexyl)diphenate and mixed heptyl, octyl and nonyl diphenates can be prepared by esterification of diphenic acid, and allege that the resultant esters are useful as plasticizers for vinyl chloride. Similarly, in an article entitled “Synthesis of dialkyl diphenates and their properties”, Shioda et al., Yuki Gosei Kagaku Kyokaishi (1959), 17, disclose that dialkyl diphenates of C1 to C8 alcohols, said to be useful as plasticizers for poly(vinyl chloride), can be formed by converting diphenic acid to diphenic anhydride and esterifying the diphenic anhydride. However, since these processes involve esterification of diphenic acid or anhydride, they necessarily result in 2,2′-substituted diesters of diphenic acid. Generally, such diesters having substitution on the 2-carbons have proven to be too volatile for use as plasticizers.
An alternative method of producing dialkyl diphenate esters having an increased proportion of the less volatile 3,3′, 3,4′ and 4,4′ diesters has now been developed. In particular, it has been found that dimethyl biphenyl compounds containing significant amounts of the 3,3′-dimethyl, the 3,4′-dimethyl and the 4,4′-dimethyl isomers can be economically produced by hydroalkylation of toluene and/or xylene followed by dehydrogenation of the resulting (methylcyclohexyl)toluene and/or (dimethylcyclohexyl)xylene product. The resultant mixture can then be used as a precursor in the production of biphenylester-based plasticizers by, for example, oxidixing the methyl-substituted biphenyl compounds to convert at least one of the methyl groups to a carboxylic acid group and then esterifying the carboxylic acid group(s) with an alcohol, such as an oxo alcohol. One important step in this overall process is the dehydrogenation reaction and, in particular, it has now been found that the addition of tin to the dehydrogenation catalyst significantly improves the selectivity to the desired dimethyl biphenyl isomer mixture.